1. Field of the Invention
The present invention relates to implantable hearing systems for assisting hearing in hearing-impaired persons. In particular, the present invention relates to improving signal quality in implantable hearing assistance systems by reducing electromagnetic interference and minimizing high frequency audio signal attenuation.
2. Description of Related Art
Some implantable hearing assistance systems use a microphone located in or near the ear to convert acoustic sound energy into an electrical signal. The electric signal is amplified, modulated and then directly communicated by a transducer to the inner ear to stimulate the cochlea to assist hearing. Alternatively, the amplified signal is communicated to a transducer for conversion to mechanical acoustic energy for vibratory application to the stapes of the middle ear or the cochlea. The microphone can be located externally, adjacent the ear, or within the external auditory canal. The transducer is commonly connected to a portion of the middle ear, known as the ossicular chain, which includes the malleus, incus and stapes. Vibrations are emitted from the transducer into and through the ossicular chain to the cochlea of the inner ear.
Electrical connections such as lead wires are used to span the gaps between the transducer and the electronics unit/amplifier. For example, FIG. 1 illustrates a prior art conventional hearing assistance system with such lead wires. System 10 is implanted into auditory system 11 and includes a sensor transducer 12, lead wires 14, and electronics amplifier unit 16 and driver transducer 18. Transducer 12 is located within the middle ear and operatively coupled to malleus 20 of the middle ear. Lead wires 14 extend from sensor 12 to electronics/amplifier 16 and then to driver transducer 18, which is operatively coupled to stapes 22.
When the length of the electrical lead wires 14 becomes significant, system 10 is increasingly susceptible to electromagnetic interference (EMI). EMI is the reception of unwanted electrical signals that are present in the environment at all times. Most EMI is caused by signals at very high frequencies, such as those used in cellular phones (e.g., 900 MHz). Under some conditions these high-frequency signals can cause low-frequency, audible, interference in electronic sound processing devices. A device""s susceptibility to EMI is related to the input impedance of the conductor receiving the EMI and to the physical size of that conductor. A large conductor with a high-input impedance will be more susceptible to EMI.
An additional problem encountered when using a high-impedance sensor is the effect of the lead capacitance which it must drive. A larger capacitance will cause high frequency audio signals to be attenuated. For example, a longer lead wire driven by a high-impedance sensor yields a large capacitance, producing high frequency audio signal attenuation.
Since very small changes in signals and acoustics mean large changes in the quality of hearing, even small amounts of EMI and high-frequency attenuation are undesirable. Moreover, with the drive to miniaturize implantable electronic components (e.g., amplifiers, filters, etc.), adding protective mechanisms to defeat EMI is undesirable as these mechanisms would add bulk, cost, and weight to the implantable components.
The importance of restoring hearing to hearing-impaired persons demands more optimal solutions in hearing assistance systems. Ideally, an improved hearing assistance system both minimizes electromagnetic interference and maximizes high-frequency performance without adding unnecessary components to produce a better acoustic signal for reception into the inner ear.
An implantable hearing assistance system includes a sensor transducer and an electronics unit. The sensor transducer, such as a piezoelectric transducer, is operatively coupled to an auditory element of the middle ear (e.g., malleus), and is electrically connected to the electronics unit. The transducer and the electronics unit are arranged together to minimize the driving impedance and lead capacitance therebetween, thereby minimizing EMI susceptibility and minimizing high audio frequency signal attenuation of the hearing assistance system.
In one example, the transducer and the electronics unit are disposed immediately adjacent each other or physically joined together to virtually eliminate (or at least significantly shorten) the length of the electrical connection between the transducer and the electronics unit. This arrangement effectively prevents high frequency audio signal attenuation associated with lead capacitance of a long-length lead wire and/or associated with a high impedance sensor that drives the lead wire. Eliminating the electrical connection or lead wire minimizes EMI susceptibility since the conductor previously susceptible to EMI has been reduced to having little or no input impedance and little or no physical size. In another example, the electronics unit is located remotely from the transducer and a preamplifier (or other impedance transforming electronics) is placed in close physical proximity to the transducer in the middle ear between the transducer and the remaining electronics unit. This arrangement transforms the impedance from the high impedance sensor to the connecting lead wire so that a significantly smaller impedance is presented to the connecting lead wire. This impedance transformation reduces high frequency audio signal attenuation. Minimizing susceptibility to electromagnetic interference and minimizing high frequency audio signal attenuation with these methods and devices enhances hearing assistance achieved by middle ear implantable hearing assistance devices.